Caught up with the discussion, overall like the changes so far for their elegance.
My opinion about the usefulness of pathfinder/scout/explorer is that they suffer from standard military units already being decent at reconnaissance with radius 2 sight for the latter. You can move your military units with relatively safety with this radius, without much risk for falling into more enemy units than you can handle. You can even just leave isolated units fortified around your empire to keep yourself informed of enemy incursions. That's a lot of recon being done already without the use of dedicated recon units.
The way I can see recon units having the intended use in military operations (a.k.a. more than exploring) is by reducing the sight of non-recon military units to 1, while having the base sight of recon units as 3. That alone would increase the usefulness of keeping recon units around. Watching foreign movements outside your borders, avoid accidentally bumping into units you had no intention to, finding enemy compositions and movement, as well as exposed flanks. A radius 1 sight unit can't perform such tasks, but even radius 2 sight units are doing right now.
It also applies to naval units in many ways, given that they can have their sight radius increased by the Great Lighthouse and Imperialism's opener. Allowing Explorers to have non-reduced sight when embarked would already increase their use greatly in a scenario where naval units aren't so good at spotting enemy fleet.
This would probably be an impactful change, since we are so used to having a 2-sight standard even for mounted units. But I think this is the most elegant way to have the three early recon units being relevant, and withotu need for a massive overhaul on the promotion lines.
Note: Before any confusion, I approve the Pathfinder - Scout - Explorer implementation, very elegant and I want it to remain.